(1) Field of the Invention
This invention relates to a process for catalytically oxidizing methacrolein in the vapor phase with molecular oxygen to produce methacrylic acid, wherein an improved catalyst is used giving an enhanced yield of methacrylic acid and exhibiting a long catalyst life.
(2) Description of the Prior Art
Many catalysts have been heretofore proposed which are used for oxidizing unsaturated aldehydes, such as acrolein and methacrolein at an elevated temperature in the vapor phase with molecular oxygen to produce corresponding unsaturated carboxylic acids, such as acrylic acid and methacrylic acid. However, proposals of the catalysts used for the oxidation of methacrolein to methacrylic acid are fewer in number than those of the catalysts used for the oxidation of acrolein to acrylic acid.
Certain catalysts were proposed as being capable of being used both for the oxidation of acrolein and the oxidation of methacrolein. It is generally accepted, however, that such catalysts exhibit reduced catalytic activity for the oxidation of methacrolein to methacrylic acid as compared with catalytic activity for the oxidation of acrolein to acrylic acid. That is, the yield of methacrylic acid is far lower than the yield of acrylic acid, provided that the same catalyst is used in the respective oxidation reactions. It is presumed that one of the reasons for which the yield of methacrylic acid is lower than the yield of acrylic acid is that methacrolein has a branched carbon chain, i.e., a methyl group, which is susceptible to oxidation, and thus, it is difficult to selectively oxidize the aldehyde group without the oxidation of the methyl group.
Heretofore, proposed catalysts used for the oxidation of methacrolein to methacrylic acid have some of the following defects: (1) the conversion of methacrolein, the selectivity to methacrylic acid and the yield of methacrylic acid are low; (2) the reaction temperature is undesirably high which influences the utility and the life of the catalyst and further causes side reactions; and, (3) the durability is poor. Typical examples of conventional catalysts for use in the oxidation of methacrolein are those which contain, as the essential elements, molybdenum, phosphorus, vanadium and an alkali metal. Illustrations of such catalysts are enumerated, for example, as in the following catalyst compositions: Mo-P-V-X-O (X=at least one metal of K, Rb, Cs and Tl; Japanese Laid-open patent application No. 82,013/1975), Mo-P-V-X-Y-O (X=at least one metal of K, Rb, Cs and Tl, Y=at least one metal of Sr, Zn, Cd, Nb, B, Pb, Bi and W; Japanese Laid-open patent application No. 123,619/1975 corresponding to U.S. Pat. No. 4,075,244), Mo-P-Cs-X-O (X=at least one metal of V, Nb and Tl; Japanese Laid-open patent application No. 135,020/1975), Mo-P-X-Y-O (X=at least one metal of V, Nb and Ta, Y=at least one metal of K and Tl; Japanese Laid-open patent application No. 65,713/1976); P-Mo-X-Y-O (X=at least one metal of K, Rb, Cs and Tl, Y=at least one metal of V, Fe, Mn, Ni, Ta, W, Sb, Co, Nb, Zn, Cd, U, Bi and Sn; Japanese Laid-open patent application No. 115,413/1976); Mo-P-X-Y-O (X is at least one metal of V, Fe, Pb and Ni, Y=at least one metal of K, Rb, Cs and Tl; Japanese Laid-open patent application No. 52,120/1976); and Mo-P-X-Y-O (X=at least one metal of K, Cs, Rb and Tl, Y=at least one metal of Ni, Sn, V, W, In, Zr and Ba; Japanese Laid-open patent application No. 57,117/1977).
Furthermore, a catalyst having the composition of Mo-V-P-O, which may optionally contain at least one metal of Bi, As, B, Ce, Cr, Ag, Fe, W, Pb, Mn, Tl, Te, Ni, Nb, B, Sn and Cu, is disclosed in U.S. Pat. No. 3,875,220. According to the working examples of this patent, the yield of methacrylic acid is not satisfactory, i.e., in the range of from 44.7% to 57.6%.